1. Field of the Invention
The present invention relates to a traverse apparatus for traversing, e.g., a sheet- or plate-like member to be traversed by rotating a rotary member such as a roller and an image recording apparatus for forming an image on a traversed member.
2. Prior Art
FIG. 9 shows a prior art image recording apparatus for optically recording characters or images on a recording sheet such as a photosensitive film. In FIG. 9, a stack of nonused recording sheets 31 are stored in a supply magazine 32. The uppermost sheet is picked up by a separation mechanism (not shown) from an opening portion 33 formed in the supply magazine 32, and is placed on a guide plate 27. The sheet placed on the guide plate 27 is traversed by a traverse means (not shown) to a position where it is sandwiched between a driving roller 12 and a driven roller 21. The driving roller 12 is directly coupled to a motor 11 and an encoder 13.
A lower portion of FIG. 9 shows a block diagram of a motor control circuit. A DC amplifier 52 is used for amplifying an error, and receives an output voltage from a constant voltage source used for setting a motor speed. A motor driving circuit 53 controls the DC motor 11. A pulse signal corresponding to a rotation signal output from the encoder 13 for detecting a rotation speed is input to a pulse-voltage converter 55. The converter 55 converts the pulse signal output from the encoder 13 into a voltage, and feeds it back to the DC amplifier 52. As can be seen from these circuit configurations, the output from the encoder 13 is compared with a setting value voltage to perform feedback control, so that the motor 11 is controlled to a given rotation speed, thus performing a sub scanning operation of the sheet.
A guide plate 28 holds a sheet 23 passing the gap between the guide rollers 12 and 21. A recording laser beam 22 emerges from an optical unit (not shown), and performs main scanning in a direction A in FIG. 9. A light-emitting element 24 and a light-receiving element 25 are used for detecting the leading end of a sheet. A hole is formed in the guide plate 28 at a position immediately below a mirror 26. In a sheet absent state, light emitted from the light-emitting element 24 is reflected by the mirror 26, and then reaches the light-receiving element 25. By utilizing this detection device, a write start timing by the laser beam is obtained
A recorded sheet is traversed inside a receiving magazine 35 from an opening portion 36 by a storage mechanism (not shown). Recorded sheets 34 are stacked and stored in the receiving magazine.
When the apparatus shown in FIG. 9 is used for recording a very fine image such as a medical image, constancy of a sub scanning velocity v of a sheet is very important, and a wow/flutter must be suppressed to about 0.2% or less.
In the prior art shown in FIG. 9, the velocity v of a sheet is not directly measured That is, a pulse count of the encoder obtained by converting rotation of the motor shaft is detected to calculate the velocity v. The velocity v is given by: EQU v=r.omega.=r.multidot.2.pi.n/N
where v is the velocity of a sheet, .omega. is the angular velocity of the motor, r is the radius of the roller 12, n is the pulse count of the encoder per unit time, and N is the pulse count per 360.degree. of the encoder.
In the control circuit shown in FIG. 9, as shown in FIG. 10, control is made to achieve .omega.=constant. Meanwhile, when the rotation center of the roller 12 does not coincide with the center of the section of the roller 12, i.e., when the roller is eccentrically mounted with respect to the rotation center, the radius r gradually changes during one revolution of the roller. Therefore, even if .omega.=constant, the traverse velocity v of the sheet periodically changes, as shown in FIG. 11. For example, assuming that a roller having a diameter of 10 mm suffers from an eccentricity of 0.01 mm, even if the section of the roller is a circle, the eccentricity causes a wow/flutter of 0.4%. Thus, it is difficult to cause a total wow/flutter including other velocity variation factors to fall within a target value. If the roller diameter is increased while the eccentricity remains the same, the influence of the eccentricity is reduced. However, it is disadvantageous for realizing a compact apparatus, and a motor speed must be decreased, resulting in difficult control. In this manner, in the prior art, the influence of the eccentricity of the roller cannot be eliminated in the sheet velocity control.